1. Field of the Invention
The present invention generally relates to printers that produce printed characters and graphics by moving a print head along a fixed line across the surface of a print medium, such as paper, and more particularly to a control method and structure for moving the print medium past the print head. The invention further relates to a mechanism for correlating the print medium advancement distance with preselected print modes.
2. Related Technical Art
One type of printer found in the art is a two-motor type printer that uses one motor as a dedicated driver for moving a carriage on which the print head is mounted, and the other as a dedicated driver for a feeding mechanism for paper or other print media. In this type of printer, each motor is controlled independently, allowing carriage movement distances to be freely selected depending only on the desired length for the printed line, and paper feeding is performed without being affected by carriage operation.
Another type of printer commonly found in the art is that of a serial printer that also drives a paper feed mechanism using the drive mechanism for the print head. In this type of printer, the paper is advanced when each carriage return occurs.
One variation on serial printers is shown in FIGS. 14 and 15, and disclosed in Japanese Laid-Open Patent Publication 3-50715 which shows a printer that facilitates adjustments to the amount of paper advance or feeding, and also adds a fast feed feature. In FIGS. 14 and 15, a printer 1 is shown using a helical cam 3, with a helical cam profile 2 formed on its surface, to drive a thermal type print head 4. Print head 4 is supported by, and moves along, a main guide shaft 7 parallel to a platen which supports the print medium. Groove-shaped cam profiles 5 and 6 are formed on guide shaft 7 for driving paper feeding operations. When print head 4 is positioned on the left end of main guide shaft 7, a pin, not shown, mounted under head 4 engages and interacts with cam profile 5 or 6, by pushing against one side of the groove-shaped profile, and causes main guide shaft 7 to rotate a prescribed amount. The rotary motion of guide shaft 7 is transmitted through a gear on the end of the shaft to a paper feeder 8 which is also rotated, advancing paper over the platen, and past print head 4.
The relative physical relationship of groove-shaped cam profiles 5 and 6 on main guide shaft 7 are illustrated in the form of "cam lines" in FIG. 15. As shown in FIG. 15, two distinct profiles, 5 and 6, are provided on the surface of main guide shaft 7, and each is configured for different angles of rotation for the shaft. When print head 4 moves to the left end of shaft 7, the head pin, acting as a cam follower, transfers between cam profile 5 and cam profile 6. That is, in a character mode in which predesigned or preformed characters are printed and a relatively large fixed media movement is used to accommodate the area prescribed for such characters, print head 4 traverses print media in the region between left end position Q1 and right end position Q3, and main guide shaft 7 is rotated through a rotational angle F.sub.1 by cam profile 5. A one-way clutch is used so that feeder 8 only rotates in one direction, the direction in which paper is fed, during movement of the carriage and does not counter-rotate when the print head moves between the Q1 and Q3 positions.
In graphics mode, however, the amount of paper movement or feeding is relatively smaller than in character mode since the image is being constructed from rows of finely spaced dots otherwise used to form characters. In this mode, head 4 reciprocates between left end Q1 and right end position Q4, and the head pin moves between cam profile 5 and cam profile 6, so that as print head 4 returns to position Q1, main guide shaft 7 is only rotated through a rotational angle F.sub.2, where F.sub.2 &lt;F.sub.1. When print head 4 moves between positions Q1 and Q2, the pin transfers along a shallow groove-shaped interconnection cam profile 9 from cam profile 6 to cam profile 5, which is relatively deeper than cam profile 6.
In addition, in this particular type of printer, fast media feeding is made possible by reciprocating head 4 between positions Q1 and Q2. In this manner, the prior art printer shown in FIG. 14 uses only one motor as a dedicated power source both for moving print head 4 and feeding paper. This structure makes it possible to obtain a compact, lightweight printer. Also, since the paper feeding distance is adjustable, this type of printer mechanism accommodates a graphics mode as well as a character mode, providing additional advantages.
However, the printers available in the art have exhibited several disadvantages. Two-motor printers require simultaneous control of the respective motors for moving the print head and feeding the paper, which complicates the printer control system and makes it difficult to achieve a compact and lightweight printer at reduced cost. Serial printers can be made compact, lightweight, and at low cost, but are difficult to add functions to so that the amount of paper fed for graphics and other modes can be switched, or for providing fast feeding, etc.
Printers using groove-shaped cams that engage a paper carriage can also experience external forces due to variations in the frictional force exerted between the pin on the print head and the sides of the groove-shaped cam follower, etc. Such forces, and variations, are transmitted directly to the print head, affecting the speed with which the print head moves, and degrading print quality. The necessity of machining a groove in a long, cylindrical, main guide shaft, requires more time and precision for machining parts, making it difficult to lower costs. The spring loaded latch in the one-way clutch, also generally causes noise when the shaft is rotated in reverse during a part of the head movement.
Furthermore, when using a spring actuated one-way clutch, the magnitude of the spring force used to urge a drive element against a driven element is very critical. If this force is too weak, sufficient amounts of rotational force may or may not be transferred. If this force is too strong, counter rotation in a direction in which the clutch should be disengaged may sometimes occur, and sound noise from any ratchet teeth might be increased. Therefore, the accuracy or precision with which paper is fed and the resulting printing quality for the prior art is not very high.